Meteorite display after Ice toil

Press, 22 June 1989, Page 15

Meteorite display after Ice toil

MiStuinWirs

Recently put on display in the Hall of Geology at the Canterbury Museum is a 63-kilogram meteorite collected in Antarctica last summer by an expedition from the museum. The meteorite is on open display so that visitors can touch it.

It was the fifth geological expedition to Antarctica supported by. the Canterbury Museum, and the four-person party consisted of Fraka Harmsen and Martin Kirkbride as geologists, Ray Waters as field leader, and myself as scientific leader and geologist. The party was airlifted 330 kilometres from Scott Base to the Darwin Glacier by C-130 ski-equipped Hercules. From this put-in point we travelled 510 kilometres, using two toboggans and four sledges to move all our equipment.

The purpose of the expedition was to study the lower part of a 1200-metre thick pile of mainly sandstone rocks of Devonian age (about 400 million years old). These rocks, originally sediments, blanket the older “basement” and have been eroded to form mountains. Shells and skeletons of animals have not yet been found in these rocks in this part of Antarctica. But animal

burrows and trackways are common, and were one of the main reasons for the expedition. Research on these rocks ties in with that of earlier museum expeditions, as well as work on rocks of similar age on the West Coast of New Zealand.

After working on both sides of the 20-kilometre wide Darwin Glacier, we sledged down the glacier to a prominent landmark known as Roadend Nunatak, then battled an icy route up the Hatherton Glacier to the Britannia Range.

Geological work in this range had previously been limited to a four-man University of Waikato party in 1978, who manhauled down the Hatherton Glacier from a put-in point near the polar plateau, and mapped glacial features and some bed rock along the valley side as they went. Most of the Britannia Range was at that time unnamed,- so the Waikato party proposed several classically derived names for the peaks and valley such as Ituna and Isca.

Derrick Peak lies near the junction of the Hathej-ton Glacier and a tributary glacier named the McCraw. When they climbed this peak in 1978, the Waikato party found an unusual rock

lying on the surface, which was so heavy that they knew it had to be an iron meteorite. Searching the wide rocky ledge 300 metres up the mountain, they discovered a further five specimens, all iron meteorites, totalling 80 kilograms.

During the 1978 summer, there had been a major United Statessupported scientific effort on the Darwin Glacier with the building of Camp Darwin. Many of the scientists working from this base were concerned with glacier movement, especially the 27kilometre wide Byrd Glacier which moves at a phenomenal 840 metres per year, making it the most active glacier in east Antarctica, as well as the largest valley glacier in the world. A

United States-Japan meteorite search-expedition was also at Camp Darwin, and when notified of the New Zealand find, visited Derrick Peak with helicopters and located 10 further meteorites. All meteorites found were composed of nickel-iron and had similar percentages of trace elements, and all had been discovered on Derrick Peak, either on the high-level bench or on the scree slope falling directly below to the Hatherton Glacier. The total weight recovered that season was 320 kilograms, and appeared to represent a single meteorite that had fragmented during entry into the Earth’s atmosphere. The 1988-89 Canterbury

Museum party established a camp near Derrick Peak on the McCraw Glacier on December 9. From this base camp we measured and sampled the rock up Derrick Peak, and on ridges further up the McCraw Glacier. Ray Waters, the field leader, was the non-geologist in the party, but was the most keen-sighted. Ray had already made an important discovery of fossil plants in Devonian rocks on a newly visited peak in the Cook Mountains, which we propose to call Gorgon’s Head, and, consequently, we asked him to “keep his eyes open” for meteorites while we were on Derrick Peak. “What does a meteorite look like?” he asked. "Dark brown like weathered dolerite, but pick it up and you’ll know it’s too heavy to be dolerite.” While the three geologists measured and studied the Beacon rocks, Ray scanned the ground looking for a rock that was different. After several hours he shouted, “I found one!” At first I though he was joking, but when he handed the rock to me, the weight alone confirmed it’s being a meteorite. Derrick Peak is a big mountain, and we needed to spend two whole days geologising on its slopes. A total of seven meteorite fragments were found lying on the surface, with only slight traces of corrosion on their undersides. The best one by far was a fragment that was found by Ray while the three others were higher bn the mountain. “It will make a superb display specimen,” he said when he joined us. "And it shouldn’t be too difficult to carry down.” It turned out to be extremely difficult. On the way back, in bright sunlight at 11.00 at night, we tried to heave the reluctant specimen into a rucksack. Once loaded, Martin tried to carry the meteorite on his back, but after 50 painful paces gave up. The specimen was not exceptionally large, but it was extremely dense, heavy, and impossible to backpack. Several days later, after a great deal of thought, we returned to the high ridge on a day when the weather was too bad to travel up the glacier. We carried in our packs a “kitset” minisledge made from a halved old sledge runner, and a wooden fuel drum holder. It was snowing, and on the ridge crest the wind was extremely strong with blizzarding snow. After an unpleasant and freezing struggle, the sledge was intricately lashed together with leather and twine. Lifting ropes were tied to each end of the two parallel runners. The meteorite was wrapped in a sack, then protected by several layers of bubble-plastic, before: being lashed on to the drum cradle in the middle of the sledge. The “homemade” sledge

was carried across the ridge, and then heaved and lowered down the steep 300-metre slope to' the Hatherton Glacier. . We slipped at times and gathered bruises, but brute force stopped the meteorite from running away downhill. The descent seemed to take an awful long time. We reached the wide, jumbled moraine at the bottom 10 hours after leaving camp. Thirst and tiredness made us deposit the meteorite and walk around the base of the mountain to the shelter of our tents. Several days later when we had finished work up the McCraw Glacier, we returned, and after carefully flagging a route through the giant heaps of gravel in the moraine, we pulled the meteorite across frozen lakes, up and down snow slopes, and over rocks and boulders until at last the edge of the Hatherton Glacier ice was reached and the meteorite could be loaded on to a motor-tobog-gan. Here it remained in a rock box until the party was pulled out of the field in January.

At Scott Base the meteorite was weighed, and found to be heavier than we imagined — 63 kilograms or 147 pounds. The total weight of all the specimens ever collected from Derrick Peak now became 391.5 kilograms. One of the smaller meteorites was especially interesting. It was collected by Martin, not from Derrick Peak, but from the Onnum Valley five kilometres to the south. If time had allowed, many more fragments might well have been found strewn over the gravel covered ground. Although more than 6000 meteorites have been collected in Antarctica, the Derrick Peak meteorite has the distinction of being the only one not to have been found on blue ice. All the other Antarctic meteorites, which originally fell on to the Polar Ice-cap thousands of years ago, have become concentrated’ after melting in blue ice areas where mountain barriers have prevented glacier flow.

Because the Derrick Peak and Onnum Valley fragments are all alike in composition, and appear to have come from a single fragmenting body, it seems more probable that they fell directly on to the mountain and its adjacent valleys as a large meteorite shower. Any associated cratering disappeared long ago. Each meteorite fragment has its own ablation pits and is enveloped by its own fusion crust, produced by heating while entering the Earth’s atmosphere, which suggests that fragmentation occurred high up in the stratosphere. The original meteorite must have been at least half a tonne in weight before fragmentation.

By

MARGARET BRADSHAW